Asian macaques develop an AIDS-like disease upon infection with lentiviruses that endemically infect African nonhuman primates. However, African monkeys fail to develop disease despite persistent and life-long infection with the same viruses. A clear understanding of the mechanisms responsible for the natural resistance of African monkeys to lentivirus-induced disease might support current strategies to develop AIDS vaccines or provide insight into new treatment modalities for HIV infection. In this proposal, we will utilize the infection of two African green monkey species infected with SIVagm to study this phenomenon. Using the models of monoclonal antibody-induced depletion of CD8+ lymphocytes and B cells which we developed previously, we will assess the role of adaptive immune responses to the natural resistance to disease seen in these species. Specifically, we will: 1. Determine the role of adaptive immune responses mediated by CD8+ lymphocytes in controlling SIVagm viremia and disease progression in African green monkeys. 2. Determine the role of humoral immune responses in affecting the course of the same infections by depletion of B lymphocytes. 3. Assess whether abolishing both cellular and humoral adaptive immune responses will affect the course of SIVagm infection of African green monkeys.